DAVIS Mark 15, 25 Sextant - Manual (Page 9)

Page 7
To adjust the Beam Converger™ horizon mirror (Mark 25 only):
1. Adjust for side error by making the Beam Converger perpendicular to the
frame.
2. Holding the sextant in your right hand, raise the instrument to your eye.
3. Look at any horizontal straight edge (the sea horizon or the roof of a
building, for example) and move the index arm back and forth using the
quick release levers.
The real horizon will remain still while the reflected horizon will appear only
when the arc and drum scales read close to zero.
4. Line up the reflected horizon and the real horizon with the knob so that
both appear together as a single straight line (Fig. 8).
Figure 8
Figure 9
5. Without changing the setting, look through the sextant at any vertical
line (a flagpole or the edge of a building, for example) and slowly tighten
or loosen the screw closest to the frame at the back of the Beam
Converger, until the real and reflected vertical lines perfectly coincide
(Fig. 9).
This is particularly easy since the two images have different colors. It is simply
a matter of putting one image exactly on top of the other.